Hydrocarbons are ubiquitous compounds in nature. The surface waxes of plants and insects contain very-long chain, non-isoprenoid hydrocarbons of 21 to over 50 carbons. Plant cuticular hydrocarbons are generally straight-chain, n-alkanes whereas insect cuticular hydrocarbons also often contain methyl-branched and unsaturated components. Long-chain hydrocarbons are also present in algae, uropygial glands of water birds, and in small amounts in many other organisms. Long-chain hydrocarbons of insects play central roles in waterproofing the insect cuticle and function extensively in chemical communication where relatively non-volatile chemicals are required. The recognition of the roles that hydrocarbons serve as sex pheromones, kairomones, species and gender recognition cues, nestmate recognition, dominance and fertility cues, chemical mimicry, primer pheromones, task specific cues and even as cues for maternal care of offspring has resulted in an explosion of new information in this area.
The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular wax layer on the cuticle. The development and application of combined gas-liquid chromatography and mass spectrometry allowed rapid and efficient analyses of insect hydrocarbons. In the late 1960s and during the next few decades, it was recognized that for many insect species, very complex mixtures of normal (straight-chain), methyl-branched and unsaturated components existed, with chain lengths ranging from 21 to 50+ carbons. It was also recognized that the variety of chain lengths, the number and positions of the methyl branches and double bonds provided insects with the chemical equivalent of the visually variable colored plumage of birds.
Insects synthesize hydrocarbons by elongating fatty acyl-CoAs to produce the very long-chain fatty acids that are then converted to hydrocarbons by loss of the carboxyl group. Methyl-branched hydrocarbons (with the exception of 2-methylalkanes) arise from the incorporation of a propionyl-CoA group (as methylmalonyl-CoA derived from valine, isoleucine or methionine) in place of an acetyl-CoA group at specific points during chain elongation. 2-Methylalkanes arise from the elongation of the carbon skeleton of either valine (even number of carbons in the chain) or isoleucine (odd number of carbons in the chain). Insect hydrocarbon biosynthesis occurs in oenocytes (large secretory cells found in clusters underlying the epidermis of larval abdominal segments).
Although it is now clear that fatty acyl-CoAs are reduced to aldehydes and then converted to hydrocarbons by the loss of the carbonyl carbon, the mechanism by which the latter step occurs remains to be identified.